The present invention relates to a cipher system and more particularly to a method and apparatus in such system in which digitally encoded data characters are selectively encrypted or deciphered in response to predetermined control characters and to a past dependent algorithm.
Information storage and retrieval systems utilizing centrally located computers with which communication may be established by practically any subscriber in a telephone system are highly vulnerable to unauthorized access. Information of a sensitive nature which is stored in computer memory banks is virtually open to snoopers who have access to the memory through data processing networks of the telephone system. Unauthorized access and reproduction of sensitive data is therefore easily achieved. Since the theft of information does not involve physical change in the original, an authorized user may be unaware initially of the theft which may only come to light after the confidentiality or value of the stolen information becomes known and adversely affects the rightful owner.
The increasing cost of postal services coupled with the uncertainty of mail delivery has established an environment for improved postal services, one form of which is "electronic mail". Established telephone systems will undoubtedly play a large part in such a system by permitting messages to be transmitted from a calling telephone subscriber station to a called station. The full benefits of such a system would probably necessitate apparatus for message conversion into digital data and high speed transmission to the receiver where corresponding apparatus would reconvert the message into cleartext form.
The advent of an electronic mail system would necessitate safeguards to prevent unauthorized acquisition of transmitted data. Cipher systems using known techniques are useful in safeguarding the data from electronic eavesdroppers.
Some digital cipher systems have been constructed with hard-wired or custom designed logic which results in apparatus that is expensive to acquire and which is inflexible in its use. The low cost of currently available microcomputer circuits now makes it feasible to economically implement digital cipher systems so as to overcome the disadvantages of the prior art. The use of software logic for instance has an immense advantage of flexibility by which future changes in the requirements of equipment can be readily incorporated through software rather than hardware changes.